In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for radio communication. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE), also referred to herein as terminals, are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to evolve the WCDMA standard towards the fourth generation (4G) of mobile telecommunication networks. In comparisons with third generation (3G) WCDMA, LTE provides increased capacity, much higher data peak rates and significantly improved latency numbers. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing (OFDM) is used in a DL transmission from a radio base station to a user equipment. Single Carrier—Frequency Domain Multiple Access (SC-FDMA) is used in an UL transmission from the user equipment to the radio base station. Services in LTE are supported in the packet switched domain. The SC-FDMA used in the UL is also referred to as Discrete Fourier Transform Spread (DFTS)-OFDM.
A basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix (cp), which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies f or subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In a time domain, LTE downlink transmissions are organized into radio frames of 10 ms, Tframe=10 ms, wherein each radio frame comprises ten equally-sized subframes, #0-#9, each with a Tsubframe=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 contiguous subcarriers in the frequency domain. The resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
Downlink and uplink transmissions are dynamically scheduled, i.e. in each subframe the radio base station transmits control information about to or from which user equipments data is transmitted and upon which resource blocks the data is transmitted. The control information for a given user equipment is transmitted using one or multiple Physical Downlink Control Channels (PDCCH). Control information of a PDCCH is transmitted in the control region comprising the first n=1, 2, 3 or 4 OFDM symbols in each subframe where n is the Control Format Indicator (CFI). Typically the control region may comprise many PDCCH carrying control information to multiple user equipments simultaneously. A downlink system with 3 OFDM symbols allocated for control signaling, for example the PDCCH, is illustrated in FIG. 3 and denoted as potential control region. However, the figure shows an example where one symbol out of the 3 possible is used for control signalling. The resource elements used for control signaling are indicated with wave-formed lines and resource elements used for reference symbols are indicated with diagonal lines. Frequencies f or subcarriers are defined along an z-axis and symbols are defined along an x-axis. After channel coding, scrambling, modulation and interleaving of the control information the modulated symbols are mapped to the resource elements in the control region. To multiplex multiple PDCCH onto the control region, Control Channel Elements (CCE) has been defined, where each CCE maps to 36 resource elements. One PDCCH may, depending on the information payload size and the required level of channel coding protection, comprise 1, 2, 4 or 8 CCEs, and the number is denoted as the CCE Aggregation Level (AL). By choosing the aggregation level, link-adaptation of the PDCCH may be obtained. In total there are a number, NCCE, of CCEs available for all the PDCCH to be transmitted in the subframe and the number NCCE varies from subframe to subframe depending on the number of control symbols n.
As NCCE varies from subframe to subframe, the user equipment needs to blindly determine the position and the number of CCEs used for its PDCCH which can be a computationally intensive decoding task. Therefore, some restrictions in the number of possible blind decodings the user equipment needs to go through have been introduced in systems today. For instance, the CCEs are numbered and CCE aggregation levels of size K can only start on CCE numbers evenly divisible by K, see FIG. 4. E.g. AL 4 can only start on CCE number 0, 4, etc.
The set of CCEs where the user equipment needs to blindly decode and search for a valid PDCCH are called search spaces. This is the set of CCEs on an AL the user equipment should monitor for scheduling assignments or other control information, see example in FIG. 5. In each subframe and on each AL, the user equipment attempts to decode all the PDCCHs that can be formed from the CCEs in its search space. If the Cyclic Redundancy Check (CRC) value checks out, then the content, i.e. the control information, of the PDCCH is assumed to be valid for the user equipment and the user equipment further processes the received information. Two or more user equipment will often have overlapping search spaces and the network has to select one of them for scheduling of the control channel. When this happens, the non-scheduled user equipment is said to be blocked. The search spaces vary pseudo-randomly from subframe to subframe to minimize this blocking probability. FIG. 5 is an exemplifying sketch showing the search space, diagonal striped, a certain user equipment needs to monitor. In total there are NCCE=15 CCEs in this example and the common search space is marked with vertical stripes.
A search space is divided to a common part and a user equipment specific part. In the common search space, the PDCCH comprising information to all or a group of user equipments is transmitted, paging, system information etc. If carrier aggregation is used, a user equipment will find the common search space present on the primary component carrier (PCC) only. The common search space is restricted to aggregation levels 4 and 8 to give sufficient channel code protection for all user equipments in the cell, since it is a broadcast channel, link adaptation cannot be used. The search spaces of the different aggregation level, m8 and m4, of first PDCCH, with lowest CCE number, in an AL of 8 or 4 respectively belongs to the common search space as described in TS 36.213, v.10.0.1 section 9.1.1. For efficient use of the CCEs in the system, the remaining search space is user equipment specific at each aggregation level.
A CCE comprises 36 Quadrature Phase-Shift Keying (QPSK) modulated symbols that map to the 36 RE unique for this CCE. To maximize the diversity and interference randomization, interleaving of all the CCEs is used before a cell specific cyclic shift and mapping to REs, see the processing steps in FIG. 6. Note that in most cases some CCEs are empty due to the PDCCH location restriction to user equipment search spaces and aggregation levels. The empty CCEs are included in the interleaving process and mapping to RE as any other PDCCH to maintain the search space structure. Empty CCE are set to zero power and this power may instead be used by non-empty CCEs to further enhance the PDCCH transmission. The PDCCH is structured into CCE 601 and then scrambled and modulated 602. The process continues by layer mapping the signals 603 and transmits the signals in a diversified manner.
Furthermore, to enable the use of 4 antenna TX diversity, a group of 4 adjacent QPSK symbols in a CCE is mapped to 4 RE, denoted a RE group (REG). Hence, in 604, the CCE interleaving is quadruplex, group of 4, based, followed by a cyclic shift 605 that is based on Cell ID, and mapping process 606 has a granularity of 1 REG and one CCE corresponds to 9 REGs (=36 RE).
There will also in general be a collection of REG that remains as leftovers after the set of size NCCE CCEs has been determined (although the leftover REGs are always fewer than 36 RE) since the number of REGs available for PDCCH in the system bandwidth is in general not an even multiple of 9 REGs. These leftover REGs are in LTE unused by the system.
The information carried on the PDCCH is called Downlink Control Information (DCI). Depending on the configured transmission mode, e.g. if a user equipment is configured in one uplink and one downlink transmission mode, and the purpose of the message, the content of the DCI varies. As an example an uplink Multiple Input Multiple Output (MIMO) transmission is scheduled using DCI format 4 and comprises the necessary information about where the user equipment shall transmit the uplink data, i.e. the resource block assignment, which precoding matrix to use, which reference signal to use etc. The corresponding downlink DCI format is format 2C. The size of each DCI format depends on the system bandwidth and reaches in these examples 66 bits for DCI format 2C.
With the introduction of cross carrier scheduling in current systems, wherein the PDCCH transmitted on one carrier is used to schedule a Physical Downlink Shared Channel (PDSCH) or Physical Uplink Shared Channel (PUSCH) transmission on another carrier, the load on the control channel will increase and there is a problem that the capacity of the control channel is insufficient leading to increased blocking of scheduled transmissions. This further leads to difficulties to increase the system throughput and may even reduce the throughput. Moreover, new mobile data applications such as social networking and over internet telephony such as Facebook, Skype and Instant messaging clients, will increase the small packet transmission with non-periodic characteristics in time. Smaller packets imply that more users must be scheduled in a subframe and each scheduling requires one use of the PDCCH. This will also increase the load on the PDCCH, which is a problem.